1. Field of the Invention
The invention relates to a viscous dampening arrangement for a taut-band suspension system used in an electrical indicating instrument of the moving coil type and, more particularly, to such an arrangement having improved vibration dampening characteristics.
2. Description of the Prior Art
Electrical indicating instruments of the D'Arsonval or moving coil types are well known and generally include two portions: a rotor to which a coil and pointer are mounted, and a stator from which the rotor is suspended and to which a permanent magnet is mounted.
Two types of suspension systems for such electrical indicating instruments are in common use. The first utilizes a combination of a pivot and jewel provided between the stator and rotor parts in order to provide a relatively low friction movement between these parts. However, this type of suspension system is relatively sensitive to vibration and its performance gradually deteriorates over time due to wear between the pivot and jewel components. The second type, called a taut-band suspension, utilizes a thin ribbon or band of metal (or other electrically conductive relatively inelastic material) anchored, under tension, between the stator and rotor parts.
Taut-band suspension systems are noted for their relative insensitivity to vibration and shock and for their long life. However, it has been found that at certain frequencies of shock or vibration, the rotor or stator component can become resonant and thereby cause the position of a pointer attached to the rotor to oscillate, thereby lowering the accuracy of any reading. In addition, under high amplitudes of shock, the rotor and stator portions may be caused to physically contact each other. This can cause anomalous motion or readings by the pointer and also leads to increased wear on the parts.
In order to overcome these problems a taut-band suspension systems it has been proposed to provide a viscous dampening fluid between the rotor and stator parts. Such an arrangement, for example, is shown in U.S. Pat. No. 3,439,273 to Sills. In Sills, annular rings or pads of a closed-cell expanded foam are provided between facing portions of the rotor and stator parts. A viscous dampening fluid is provided therebetween and is held in position because of the high surface tension provided by the irregular surfaces of the foam pads. The viscous dampening fluid acts as a shock absorber and a vibration dampener when the instrument is subjected to shock or vibration. However, the Sills arrangement suffers from the disadvantage that it can only provide cushioning for shock or vibration along the longitudinal axis. If the vibration or shock has a component which is not parallel to the longitudinal axis the viscous dampening arrangement will not function properly since there is nothing to constrain the movement of the parts in a direction perpendicular to the taut-band. In addition, the large contact area between the viscous dampening fluid and the pads mounted on the stator and rotor parts drastically impedes the response time of the instrument.